Excess estrogenic substances in rodent diets may influence growth, diseases, tumor rates, reproductive performance, and the results of estrogenic and carcinogenesis studies. We have previously shown that rodent diets differ significantly in estrogenic activity and that a standardized diet with minimal estrogenic activity would be desirable for studies that are influenced by exogenous estrogenic substances. Recently, we compared the estrogenic activity in three diets (A,B,C) formulated to reduce the level of phytoestrogens. Our results confirmed that the three new diets, significantly differ in estrogenic activity, and suggest that these three diets cannot be used interchangeably for comparative estrogenic studies. We concluded that a standardized open-formula diet, ideally free of estrogenic substances, is essential when comparing results from comparative estrogenic or carcinogenesis studies. Such a diet can be formulated to contain less than detectable levels of the phytoestrogens by omitting the soybean meal and alfalfa meals.